The present invention is related to temperature measurement, and more particularly to temperature measurements using a transistor or diode as a sensor.
Temperature measurement using a transistor as a sensor is a common application in the semiconductor area. Such a temperature measurement is done by applying two different currents to the transistor each resulting in a respective base-emitter voltage. The difference between the two base-emitter voltages is proportional the absolute temperature of the transistor. To assure an accurate base-emitter voltage measurement, a settling period after application of an excitation current is required before sampling the corresponding base-emitter voltage. This settling time depends upon the magnitude of the applied excitation current and any filter capacitance and series resistance in the circuit. Thus, the settling time will typically be different for each applied excitation current.
Turning to FIG. 1a, a simplified prior art temperature measurement system 100 is depicted. Temperature measurement system 100 includes a temperature calculation system 140 that is electrically coupled to the base and emitter of a transistor 144. Transistor 144 is electrically coupled to a variable current source 142 that allows for exciting transistor 144 using two different currents. Temperature calculation system 140 measures the base-emitter voltage of transistor 144 corresponding to the two different current excitations applied via variable current source 142. As previously noted, the difference between the two different base-emitter voltages is proportional to the absolute temperature of transistor 144. The following equation defines the relationship between the difference between base-emitter voltage measurements and absolute temperature:ΔVbe=Vbe2−Vbe1=n*kT/q*ln(I2/I1).The ‘n’ term is known as the non-ideality factor or emission coefficient is assumed to be a constant (n=1.008) for diodes and transistors.
Temperature measurement system 100 is clocked by an oscillator 110 which has its output divided by a divider circuit 120. The output of divider circuit 120 is a clock 190 that is used to synchronize the operation of temperature measurement system 100 and in some cases other circuitry associated therewith. Various periods such as, for examples sampling periods required to sample and/or process base-emitter voltages from transistor 144 are governed by one or more period counters 130 as are known in the art.
As shown in a timing diagram 155 of FIG. 1b, a sample period 150 is paced by the slowest settling time associated with an applied excitation current. In operation, sample periods 150 each include the same predetermined number of cycles of clock 190 as counted by period counter 130, with the number of cycles being selected to match the slowest settling time. Each sample period 150 is used to sample a base-emitter voltage corresponding to a different excitation current applied by variable current source 142. Each time a different excitation current is applied to transistor 144, a delay period must be awaited to assure that the base-emitter voltage of transistor 144 to be sampled is stable. As shown, a required sample period 180 corresponds to one excitation current offering the slowest settling time, and thus utilizes the entire sample period 150b. In contrast, a required sample period 160 associated with a faster settling time utilizes only a portion of sample period 150a. In this case, the remaining portion of sample period 150a is a wasted period 170. Where, for example, wasted period 170 is the same length as required sample period 160, a twenty-five percent bandwidth overhead is incurred. The aforementioned bandwidth overhead results in a number of unused cycles of clock 190 propagating through various circuitry including temperature calculation system 140, and the corresponding unnecessary power dissipation associated therewith.
Thus, for at least the aforementioned reasons, there exists a need in the art for advanced systems and devices for temperature measurement.